Pulsing control match network and generator

ABSTRACT

A method of controlling a radio frequency processing system, the method including determining an end time of a radio frequency pulse; stopping a load applied to the radio frequency processing system based on the end time of the radio frequency pulse; adjusting an additional load having a predetermined impedance applied to the radio frequency processing system in response to the determined end time; determining a start point of a second radio frequency pulse; and stopping the additional load before the second radio frequency pulse occurs.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application. No.62/963,426 filed Jan. 20, 2020, the contents of which are incorporatedherein by reference.

BACKGROUND

Radio frequency (RF) Plasma-enhanced processing is extensively used insemiconductor manufacturing to etch different types of films, depositthin films at low to intermediate processing temperatures and performsurface treatment and cleaning. Characteristic of such processes is theemployment of a plasma, i.e., a partially ionized gas, that is used togenerate neutral species and ions from precursors inside a reactionchamber, provide energy for ion bombardment, and/or perform otheractions.

Non-uniform plasma densities within a reaction chamber may cause unevenetch rates or certain characteristics across a substrate. In certainsystems, monitoring plasma density uniformity within a reaction chamberoccurs with probes. Such probes may be exposed to the plasma environmentrely on coatings and may use active electronics to infer plasma density.Such systems may take milliseconds or more to respond to changes in theplasma. Emission spectroscopy may also be used to determine the profileof plasma density within a reaction chamber, but such system may requiremultiple lines of sight through the plasma and use complicated analysisto infer non-uniformity. Neither of these techniques are sensitive andfast enough to effectively resolve the non-uniformity issues and mayfurther be costly to implement.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying Figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a side view schematic representation of a RF plasma processingsystem according to embodiments of the present disclosure.

FIG. 2 is a schematic side view of a plasma chamber with high bandwidthsensors mounted in various positions on the electrodes in accordancewith embodiments of the present disclosure.

FIG. 3 is a cross-sectional view of a dual plate electrode assemblyhaving sensors providing voltage signals through electrical connectorshaving low shunt capacitance to electrical ground, according toembodiments of the present disclosure.

FIG. 4 is a cross-sectional view of a pedestal with an embedded highbandwidth voltage sensor, according to embodiments of the presentdisclosure.

FIG. 5 is a schematic side view of a pedestal, according to embodimentsof the present disclosure.

FIG. 6 is a top view of the propagation of axisymmetric surface wavesacross a pedestal where the plasma in the reaction chamber isaxisymmetric, according to embodiments of the present disclosure.

FIG. 7 is a top view of transverse electromagnetic surface wavepropagation across an electrode, according to embodiments of the presentdisclosure.

FIG. 8 is a top cross-sectional view of azimuthally (about the chambersymmetry axis) mounted sensors on a reaction chamber, according toembodiments of the present disclosure.

FIG. 9 is a side cross-sectional view of azimuthally mounted sensors onelectrodes, electrode bases, a top dielectric plate, viewports anddielectric wall of a reaction chamber, according to embodiments of thepresent disclosure.

FIG. 10 is a side cross-section view of a capacitive coupled plasmareactor chamber with some sensor array locations according toembodiments of the present disclosure.

FIG. 11 is a side cross-section view of a model inductive plasma reactorchamber according to embodiments of the present disclosure.

FIG. 12 is a schematic side view partial cross-section of a RF plasmaprocessing system with some possible sensor locations according toembodiments of the present disclosure.

FIG. 13 is a schematic partial cross-section including the dielectricwall of a RF plasma processing system where the sensors are mounted onthe dielectric surface proximate the inductive coupling antennaaccording to embodiments of the present disclosure.

FIGS. 14, 15, and 16 are process phase diagrams for a RF plasmaprocessing system matching network according to embodiments of thepresent disclosure.

FIG. 17 is a schematic representation of the phases of a radio frequencyprocessing system matching network according to embodiments of thepresent disclosure.

FIG. 18 is a graphical illustration of voltage at a beginning of a radiofrequency pulse in a radio frequency plasma processing system, accordingto embodiments of the present disclosure.

FIG. 19 is a cross-sectional view of a RF processing system according toembodiments of the present disclosure.

FIG. 20 is a RF and DC decay process diagram for a RF plasma processingsystem matching network according to embodiments of the presentdisclosure.

FIG. 21 is a schematic representation of where a switchable component toadd an additional load may be located according to embodiments of thepresent disclosure.

DETAILED DESCRIPTION

Illustrative examples of the subject matter claimed below will now bedisclosed. In the interest of clarity, not all features of an actualimplementation are described in this specification. It will beappreciated that in the development of any such actual implementation,numerous implementation-specific decisions may be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a developmenteffort, even if complex and time-consuming, would be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Further, as used herein, the article “a” is intended to have itsordinary meaning in the patent arts, namely “one or more.” Herein, theterm “about” when applied to a value generally means within thetolerance range of the equipment used to produce the value, or in someexamples, means plus or minus 10%, or plus or minus 5%, or plus or minus1%, unless otherwise expressly specified. Further, herein the term“substantially” as used herein means a majority, or almost all, or all,or an amount with a range of about 51% to about 100%, for example.Moreover, examples herein are intended to be illustrative only and arepresented for discussion purposes and not by way of limitation.

Turning to FIG. 1, a side view schematic representation of a RF plasmaprocessing system 100, according to embodiments of the presentdisclosure is shown. RF plasma processing system 100 includes a first RFgenerator 105 and a second RF generator 110, a first impedance matchingnetwork 115, a second impedance matching network 120, a sheath 125, aplasma powering device, such as showerhead 130 or equivalent poweredelement such as an electrode, and a pedestal 135. As used herein, plasmapower devices may refer to any device that introduces power to generateplasma and may include, for example, showerhead 130 and/or other typesof electrodes, as well as antenna and the like.

RF plasma processing system 100 may include one or more RF generators105, 110 that deliver power to a reaction chamber 140 through one ormore impedance matching networks 115, 120. RF power flows from the firstRF generator 105 through the impedance matching network 115 into plasmain reaction chamber 140 to showerhead 130 or sidewall, to an electrodeother than showerhead 130, or to an inductive antenna (not shown) thatelectromagnetically provides power to the plasma, where after the powerflows from the plasma to ground and/or to pedestal 135 and/or to secondimpedance matching network 120. Generally, first impedance matchingnetwork 115 compensates for variations in a load impedance insidereaction chamber 140 so the combined impedance of showerhead 130 andfirst impedance matching network 115 equal the output impedance of firstRF generator 105, e.g., 50 ohms, by adjusting the reactive components,e.g., variable capacitors, within first impedance matching network 115.The term “about” is acknowledgement that, in practice, some imprecisionrelative to the range may be experienced and yet obtain satisfactoryresult. Such imprecision may result from, for example, a loss ofcalibration or drift during operation. In these situations, however, theexpressed range is the nominal target for operational conditions when inuse.

In certain examples, first RF generator 105 may provide power at a RFfrequency between about 400 KHz and 150 MHz, while second RF generator110 connected to pedestal 135 may supply power at a RF frequency lowerthan that of first RF generator 105, however, in certainimplementations, second RF generator 110 may not supply power at a RFfrequency lower than that of first RF generator 105. Typically, thefrequencies of first and second RF generators 105, 110 is such thatfirst RF generator 105 is at a RF frequency that is not an integermultiple, nor integer fraction, of the frequency of second RF generator110.

Impedance matching networks 115, 120 are designed to make adjustments totheir internal reactive elements such that the load impedance matchesthe source impedance. Generally, low reflected power is consideredpositive, however, embodiments of the present disclosure ensure that thedelivered power is maintained in reaction chamber 140, and that powerthat is reflected back towards first and second RF generators 105, 110,and that even when the reflected power is relatively high, theassociated impedance matching networks 115, 120 may monitor forward andreflected power to and from reaction chamber 140 and, using motor-driversystems, make adjustments to adjustable reactive elements, e.g., vacuumvariable capacitors. Impedance matching networks 115, 120 may containcircuitry to measure phase and magnitude of signals to determine thelevels of forward and reflected power from the intended load. As such,embodiments of the present disclosure may be effective even when theamount of reflected power is high. If there is a significant amount ofreflected power at a primary frequency, capacitors are varied until thereflected power is minimized, for example to less than about 5 Wattsand/or less than about 1 percent for the period, or in certainembodiments, to less than 1 Watt. Typically, harmonic frequency signalsare not measured, including the reflected power at harmonic frequencies.

Although RF plasma processing systems 100 have many advantages, theyhave historically been challenged to maintain control of plasma densitythroughout a multi-step process. Design tolerances on the order of onepercent non-uniformity, for example, with a density range of the sameorder relative to a nominal value remain a challenge. Achieving optimalintegrated circuit (IC) yields on each and every wafer as the featuresize shrinks below about 10 nm and the layer thicknesses are less than50 nm requires progressively tighter control of the uniformity of theplasma and neutral species to the 1% level and even less. Non-uniformplasma densities, or average densities deviating from the desired valueby more than a desired range within reaction chambers may be caused byslow changes in the chamber, changes in the RF circuit, or the rapidgrowth (on the order of less than a millisecond) of parasitic orsecondary plasmas which can lead to non-uniform ities of nano-scalefeatures across a processed wafer due to uneven etch rates.

Because even a difference in an etch rate of one percent across a wafercan cause yield problems for advanced technologies, and because it oftentakes a substantial amount of time to complete wafer processing to seethe yield loss, a need exists to promptly and accurately detectnon-uniform plasma densities or plasma density that deviates from thedesired range within a reaction chamber in a time period that may needto be less than about 1 millisecond to avoid irreversible deviations onthe wafer from desired feature profile.

Those of ordinary skill in the art will appreciate that electromagnetic(EM) surface waves may propagate on surfaces within an RF powered plasmain reaction chamber 140. These surface waves will have appreciableenergy at both the fundamental RF drive frequency and RF harmonics. Theharmonic waves' average power and power distribution are sensitivefunctions of plasma density and non-uniformity. Herein, a harmonic waveprofile is defined as the spectrum of surface waves having frequenciesthat are integer multiples of the fundamental drive frequency for an RFplasma-based reaction chamber 140. For example, if 2 MHz RF drive poweris provided to reaction chamber 140, the injected power will generatesurface waves at that frequency that propagate along interfaces betweenplasma and interior reaction chamber 140 surfaces. Harmonic surfacewaves of integer multiple frequencies may also be generated. Forexample, 2 MHz electromagnetic waves may generate 4, 6, or 8 MHz surfacewaves. Both odd and even harmonics (2nd, 3rd, 4th, 5th, etc.) mayappear, but in some examples the odd harmonics may be dominant.

Aspects of the present disclosure may provide sensor locations on andabout reaction chamber 140 and components thereof that may allow fordetecting and analyzing RF surface waves to find amplitudes and phasesof fundamental and harmonics at a plurality of points within or adjacentreaction chamber 140. The waves may be detected by sensing the rfvoltage or rf current at fundamental and harmonic frequencies on thesurface of a chamber component. In some embodiments a sensor for voltagewill include a pickup that is configured at, or proximate to, thesurface of the electrode, pedestal base, chamber wall, or strap, and aconducting line that conveys the signal from the pickup to a connectoror cable. A current sensor may include a conducting element that mayinclude one or more loops or partial loops or a linear conductor, whereone end of the conducting element is at a reference potential that maybe local electrical ground.

A plurality of sensors, e.g., two or more, may be positioned uponcertain chamber component, which will be discussed in detail below, atdifferent angles about a chamber symmetry axis for measuring the surfacevoltage or current associated with such surface waves. Herein, an anglemeasured about the symmetry axis from a reference point of the chamberis defined as an azimuth. In some embodiments such sensors may bepositioned at approximately the same distance from the symmetry axis ofthe chamber.

Sensors may be mounted in various locations on or about reaction chamberand/or components thereof. For example, sensors may be mounted on thesurface of an electrode, such as pedestal 135 and/or showerhead 130.Sensors may also be mounted on a base of an electrode either within thevacuum or outside the vacuum environment. Sensors may be mounted insidethe chamber on one or more metal wall surfaces of the reaction chamber140, and inside or outside wall areas that contain a dielectricmaterial, or on an antenna that may be used for inductively providingpower into the plasma. Sensors may also be placed on a passive antennathat may be used for sensing the EM waves proximate the boundary of theplasma or upon or proximate a plurality of conducting busses or strapsconnecting first or second impedance matching network 115, 120 to anelectrode, such as pedestal 135 and/or showerhead 130, antenna, or othercomponents that transmits power to plasma within reaction chamber 140.Sensors may also be connected to an electrical ground. The sensors maythereby pick up signals from different parts of RF plasma processingsystem 100 as they propagate on respective component surfaces.

A spectrum of RF harmonic waves is generated at the electrode-plasmainterface, e.g., sheath 125 in FIG. 1 and waves propagate in alldirections so that both amplitudes and phases of all wave componentswill vary with location on an electrode or support base. Such waves alsopropagate along the inner surface of a metal wall adjacent the plasmaand propagate through any dielectric wall that may be adjacent plasma.Such wave amplitudes and phases change in response to changes of theplasma, e.g., plasma density and non-uniformity, with response times onthe order of or less than a few microseconds. Further, the frequency andphase distributions of RF harmonic surface waves that propagate on theelectrode-plasma interface determine the frequency and phasedistributions of harmonic surface waves that propagate on the surface ofan electrode base toward impedance matching networks 115, 120, onsurfaces connected to an electrode or plasma-wall interface, or onwalls. The amplitudes and phases of the signals for fundamental andharmonic waves at the different sensor locations permit determination ofwhat part of the total EM wave field for each frequency is azimuthallysymmetric and what part is non-symmetric.

In the case of inductive plasma, signals from the plasma, e.g.,fundamental and harmonic, may propagate back to an antenna and then tothe impedance matching network feeding power to the antenna. Thefrequency and phase distributions of both fundamental and harmonic RFwaves may be monitored on a microsecond or faster timescale usingsensors mounted on such surfaces and may be compared with specifiedranges and phase relationships as indicators of plasma asymmetry orchanges in plasma density or electrical conductivity. Signals from suchsensors may be transmitted, by cables, or otherwise, to detectors thatanalyze the signal's component frequencies to produce the amplitude andphase values for each frequency component at each sensor location.

In certain implementations, the amplitudes and phases of the detected RFharmonic components may be rapidly determined by circuits (detectors) ina signal analysis compartment that may be a separate metal box orchassis, or that may be within or connected to or part of impedancematching networks 115, 120. Such amplitudes and phases may be used todetermine the status, including the radial distribution and theasymmetry of plasma by applying algorithms and plasma non-uniformitycalibrations. The signals from the sensors may be Fourier analyzed bydedicated circuits (detectors) that are fast enough to perform virtuallycontinuous spectrum analysis, updating as frequently as possible andyielding a high-rate data stream. For example, for plasma power at 13.56MHz, 512 periods may take less than 50 microseconds to process throughFourier analysis, and for pulsed plasmas when each element of the pulseoccurs at 5 KHz, this allows for updates of plasma status at a rate of10 KHz.

The results of the dedicated Fourier analyses of fundamental andharmonic waves may be stored on a separate storage medium that may beread and/or written to by an analysis processor associated with thesignal analysis compartment. Either stored results or a real-timesignals may be routed to high speed computational processors todetermine asymmetry parameters for each of fundamental and harmonicwaves. The asymmetry parameters may be compared to values previouslystored on the separate storage medium (or on a different storage medium)using algorithms (which may also be stored on the separate storagemedium or on a different storage medium) to very rapidly recognize a“Plasma Fault” condition. The analysis processor then may transmit anappropriate command, e.g., to continue the process under the presentconditions, or to make needed changes in the process conditions, tofirst and second RF generators 105, 110, and in certain implementations,more than just two RF generators and when appropriate to the impedancematching networks associated with these generators. In certainimplementations, three, four, or more RF generators may be used. Firstand second RF generators 105,110 may then continue, stop, change thepower provided, or respond in some other suitable way—for example, goinginto a reduced-power mode or a pulsed mode, or ordering certaincorrective actions, e.g., alarm triggering, power interruption, etc., toavoid improper wafer processing during a Plasma Fault or otherunacceptable situation.

The location of sensors for detecting (electric and magnetic fields of)and characterizing surface waves may, in some embodiments, be onperipheral surfaces (bare or covered by dielectric) of the pedestal 135outside the area covered by the wafer. For example, if reaction chamber140 is to process circular wafers of radius 150 mm, the pedestal-mountedsensors may be located at radii greater than 150mm from the wafer centerwhich may in some cases be under an annular peripheral dielectric forcontrolling edge effects. Sensors may additionally or alternatively belocated on the surface or periphery of the showerhead 130 facing thewafer or on the surface of the base of the pedestal 135 or the base ofshowerhead 130, whether these locations are within or outside theevacuated process environment. Sensors may also be located at variousother locations, which will be discussed in detail below, and maymonitor continuously or periodically to provide uniformity of theprocess plasma.

Using sensors outside the evacuated process environment, e.g., in thestraps or busses connecting the base to one or more of impedancematching networks 115, 120, the base of pedestal 135 and/or showerhead130, may not require passing signals through a vacuum feedthrough orinstalling transmission cables within the evacuated volume of reactionchamber 140. Sensors in such locations may monitor the fundamental andharmonic EM waves substantially continuously. This may enable an RFplasma processing system 100 to continuously provide plasma densityuniformity and determine within a very brief time whether a faultcondition has occurred or whether proper wafer or substrate processingcontinues.

In certain example implementations, the present disclosure may provideapparatuses and methods for detecting deviations of the plasma from therequired “process window” within an RF plasma processing system 100. TheRF plasma processing system 100 may include reaction chamber 140, whichmay include showerhead 130 for injecting reactant gases into reactionchamber 140, and which may also include a wafer-support pedestal 135.However, in other implementations, showerhead 130 may not inject gasinto reaction chamber 140. In some embodiments showerhead 130 may bemounted with its center near the approximate symmetry axis of reactionchamber 140 and equipped with a plurality of sensors positioned atselected azimuths around the symmetry axis. Additionally oralternatively, such sensors may be positioned on the wafer-facingsurface in peripheral areas of showerhead 130 to detect and measurepropagating EM surface waves while wafers are being processed.

Further, in some embodiments, there may be a plurality of sensors thatare mounted on the outer surface of the wafer support pedestal 135,outside the area occupied by the wafer, for detecting both amplitude andphase of the RF harmonics and fundamental surface waves. Such sensorsmay be exposed to plasma or may be covered by dielectric, ordielectric-and-metal, covers. Additionally or alternatively, sensors maybe situated on the periphery of the pedestal 135 base, within or outsidethe evacuated volume and/or below the plane defined by the wafer. Insome implementations, the sensors may be positioned on the pedestal baseto detect surface electromagnetic waves propagating toward or away fromthe wafer-supporting area of the pedestal and on the surface of thepedestal base. In certain embodiments, the sensors may be mounted closeto the wafer plane (e.g., less than 10 centimeters).

Alternatively, sensors may be mounted on a part of pedestal 135 that ismetal or another electrically conductive material and located outsidethe evacuated region of reaction chamber 140 in atmospheric conditions.Sensors located outside the evacuated region may be mounted on a regionof pedestal 135 at a radius from the pedestal symmetry axis that is atleast 50% of the maximal pedestal 135 radius, or even more than 75% ofthe maximum pedestal 135 radius. Such sensors may be positioned close—insome embodiments within a few centimeters of the vacuum seal for thesupport pedestal 135, e.g., the O-ring. In some embodiments, the totalof radial and axial propagation distance from the edge of the wafer tothe sensors may be less than about 25 cm, or in some embodiments lessthan about 15 cm, or even about 10 cm. Specific locations andorientations of sensors according to embodiments of the presentdisclosure will be discussed in detail below.

Turning to FIG. 2, a schematic side view of a plasma chamber with highimpedance sensors mounted in various positions on the electrodes inaccordance with embodiments of the present disclosure is shown. Each oftwo components serving as electrodes, i.e., pedestal 235 and showerhead230, or equivalent other powered element may use a separate RF generator205 or 210 and impedance matching networks 215 and 220. Alternatively,an electrode may have a plurality of generators and matching networksfeeding power thereto. Arrows 245 along the surface of pedestal 235indicate the inward radial direction of RF current and power flow fromthe bottom (bias) RF generator 210 that is connected electricallythrough the impedance matching network 220 to pedestal 235. The electricfield created contributes to the formation of a plasma (not shown)between the electrodes and a radial outward counterflow of current andpower, indicated by arrows 250, along the lower surface of showerhead235 or other powered element and ultimately to a selective groundingcircuit in impedance matching network 215 for showerhead 230 or theother powered element.

In certain embodiments, reaction chamber 240, having RF power from firstand second RF generators 205, 210 and impedance matching networks 215,220 may include sensors 255 on the periphery of pedestal 235 that may becovered by dielectric 260. Communication lines 265 may transmit signalsfrom each of sensors 255, that in some embodiments may be approximatelyequidistant from the pedestal symmetry axis, to Fourier analysiscircuits (not shown) that compute amplitude and phase of bothfundamental frequency and harmonic frequency surface waves picked up byeach sensor 255.

In some implementations, the Fourier analysis circuits may calculatemagnitudes and phases of the fundamental and higher order harmonics ofthe periodic, electro-magnetic surface waveforms. The resulting seriesof magnitudes, known as a Fourier series, and their phases results fromthe relation between a function in the domain of time and a function inthe domain of frequency.

Further, some of the embodiments of the disclosed matching network 220may contain a signal analysis compartment 275 or appendage of thematching network 220 that is separate and RF isolated from the RF powerhandling and impedance matching circuitry or components of the matchingnetwork 220. The signal analysis compartment 275 may contain Fourieranalysis circuit(s) (detectors) for analyzing sensor signals andyielding digital amplitudes and phases of RF fundamental and harmonicwaves. Signal analysis compartment 275 may also contain high speeddigital logic or computation processors for analyzing the relativemagnitudes and phases of signals at harmonic frequencies and derivingquantitative parameters that characterize the relative magnitudes ofaxisymmetric and non-axisymmetric harmonic components at each frequency,and their relative phases. Furthermore, in some embodiments, thedisclosed matching network 220 may be connected via a very fast networkto the second RF generator 210 as well as the controller (not shown) forthe reaction chamber 240 or RF plasma processing system 200 whereinsensors 255 are located. In some embodiments the disclosed enhancedimpedance matching network 220 may be capable of sending commands to thefirst RF generator 205 as well as communicating its calculatedparameters to the processing chamber controller and/or to the toolcontrol system.

In addition, another, first RF generator 205 and impedance matchingnetwork 215 may also be electrically coupled to the other electrode,which may be a showerhead 230 in reaction chamber 240. In oneimplementation, the first RF generator 205 may operate at differentfrequency than second RF generator 210, and its frequency may not be aninteger multiple of the frequency of the second RF generator 210.

Similarly, impedance matching network 215 monitors the reflected powerfrom the electrode and processing chamber 240 and may make adjustmentsif there is significant reflected power from the electrode. In someembodiments, the second RF generator 210 may be a 400 KHz RF generator,a 2 MHz RF generator, or a 13.56 MHz RF generator or other, while thefirst RF generator 205 may operate at a somewhat higher frequency. Insome embodiments, first RF generator 205 may operate at a frequencygreater than 25 MHz, such as 60 MHz, 100 MHz, or higher.

In one embodiment, the primary function of the first RF generator 205may be to power the reaction chamber 240 to generate plasma between theshowerhead 230 or another power source such as an electrode and pedestal235, both to generate reactive chemical species such as fluorine orchlorine and to cause ions from the generated plasma to accelerate andstrike a wafer disposed on the pedestal 235.

Disposed on the upper electrode surface, i.e., showerhead 230, facingthe lower electrode, i.e., pedestal 235, may be a set of sensors 280having bandwidth greater than about 10 times the frequency of thehighest frequency RF generator connected to that electrode. In someembodiments, each of these may have an impedance greater than about 100Ohms, and in some embodiments greater than 500 Ohms. Sensors 280 may bevoltage or current sensors or may combine both capabilities in a singlepackage—for example, where a current sensor may include one or moresegments of wire that may be covered by an electrostatic shield.

In some embodiments, the sensors 280 have electrical connections toFourier analysis circuits in the signal analysis compartment 285 of theimpedance matching network 215. The Fourier analysis circuits may outputamplitude and phase of the different frequency components from each ofthe sensors 280 and compare them with other sensors 280 and/or withreference levels that are stored in memory. The analyses of the signalsin some embodiments may include pattern recognition of amplitudes orphases or both, or artificial intelligence (AI) employing learningalgorithms that may use neural networks or conventional digitalalgorithmic processing of the signals from the sensors 280.

Signal processing by the Fourier analysis circuits to find fundamentaland harmonic component signals, both amplitudes and phases, may be donewithin less than about 10 micro-seconds and in preferred embodiments a 1microsecond or less for each of the sensor signals. The isolated signalanalysis compartment 285 of the impedance matching network 215 mayincorporate at least one computation or logic processor havingsubstantial computational capability with very high speed (<1 ns cycletime) circuits employing very high-speed logic ICs. In some embodiments,the processors in the signal analysis compartment 285 are programmableso that suppliers or users of the processing chambers 240 may provide orimplement proprietary algorithms or analytical software upon thecomputing “platform” provided in the impedance matching network 215.

In some embodiments, the software programs for calculating parametersfrom signal amplitudes and phases, and further logic algorithms fordetermining the effect on processing uniformity of excursions fromacceptable plasma conditions, may reside on a removable “plug-in”component that contains data storage and connects to the signalprocessing compartment. This software or logic calculates the extent ofan excursion of the RF electromagnetic surface wave spectrum from thatcharacteristic of nominal or proper operating conditions. Based on this,a processor associated with a controller may “decide” upon correctiveaction or termination of the process within about a millisecond, beforethe wafer is misprocessed. In some embodiments, a quantitative judgementas to the expected effect of the excursion on process uniformity orother properties may be done within about 500 micro-seconds ofoccurrence so that remedial action may start within a millisecond.Further, action may be taken such that there is minimal or no damage tothe wafer or substrate being processed in the reaction chamber 240 atthat time and thereby avoid loss of yield on that wafer or substrate.

The assessment and/or decision made in the signal analysis compartment285 of the impedance matching network 215 may, in some embodiments beperformed by the very fast computation or analytical system usingalgorithms residing on a plug-in storage and/or detachabledata-processing device. In still other embodiments, the assessmentdecisions made in signal analysis compartment 285 may be performed usingan analog or neural net type processor. Such decision may further use adecision algorithm that may reside on the detachable storage orprocessing device. The order for corrective action may then be promptlytransmitted by high speed data line from compartment 275 of theimpedance matching network 215 to the RF generator 205, which maytemporarily interrupt, change, or terminate power to the plasma. Thisassures that factory management may promptly take or plan correctiveaction for that processing chamber 240 and RF plasma processing system200.

Also shown in FIG. 2 is a set of sensors 290, which are configured onthe outer surface of the base 295 of the showerhead 230, outside thevacuum region within the reaction chamber 240 in atmospheric conditions.In some embodiments, additional sensors 296 may be mounted on thepedestal base 297 and connected by high speed signal cables to thedisclosed signal processing compartment 275 of the impedance matchingnetwork 220, as with sensors 290. The sensors 296, being located outsidethe vacuum environment of the reaction chamber 240, are substantiallyless expensive and less difficult to integrate into the information andprocessing network since no vacuum feed- is required.

Sensors 255 may be disposed in some configurations to sense voltageand/or current on the surface of pedestal 235 and may be covered andprotected from plasma by a dielectric cover 260. Sensors of this typeand location are proximate to the wafer and/or substrate and thereforemay have a sensitivity advantage in detecting certain modes of EMsurface waves that are indicative of plasma asymmetry—which is animportant type of plasma non-uniformity. These in-chamber sensors 255may use communications lines that pass through the vacuum wall via afeed-through or a in some embodiments use wireless communication linksoperating at optical or at lower frequencies.

In general, phase and amplitude patterns of each frequency of EM surfacewave over the surface of showerhead 230 and pedestal 235 may bedetermined by analysis of the signals from any of the groups of voltage,current, phase, or combination sensors 255, 280, 290, and 296, Ingeneral, EM surface waves at a given frequency produce voltage andcurrent signals having phase relationships with signals of otherfrequencies. The magnitude of the voltage at each frequency and eachpoint is the sum of voltages from all waves of that frequencyoriginating from all points across the electrode surface. For anaxisymmetric electrode surface where the power is fed symmetrically, andthe plasma is axisymmetric, axisymmetric surface-wave modes will resultfrom the superposition of waves from all parts of the electrode andother surfaces in the reaction chamber 240. In general, perfectlysymmetrical plasma in a symmetrical chamber with a symmetrical electrodecentered on the chamber's axis of symmetry would predominantly havesymmetrical lines of equal phase and amplitude in the form of circlescentered at the center of the pedestal 235.

Turning to FIG. 3, a cross-sectional view of a dual plate electrodeassembly having wide bandwidth sensors providing voltage signals throughelectrical connectors having low shunt capacitance to the surroundingarea of the electrode and to electrical ground, according to embodimentsof the present disclosure is shown. In some embodiments an electrode,such as the showerhead 330, may include two conducting plates 331, 332that are configured approximately parallel, with centers aligned, havinggenerally the same shape as the substrate or wafer. A surface of thefirst plate 331 that faces away from the second plate 332 may be exposedto the vacuum environment and to the plasma. The first plate 331 isseparated from the second plate 332 a distance that is the length of thedielectric standoff supports 333. The first plate 331 may have embeddedsensors 334, whose pucks or pickups are of conducting material and whosesurfaces are approximately co-planar with that surface of the firstplate 331 that faces away from the second plate 332.

In some embodiments, the sensors 334 may be mounted into the first plate331, surrounded by a dielectric 336 with a low dielectric constant, suchas quartz or some other suitable material. In some embodiments, thedielectric 336 may have a dielectric constant less than 5 and in someembodiments the dielectric constant may be less than 2 for inorganicmaterials such as aerogels based on quartz. The sensors 334 may havehigh bandwidth extending from 100 kHz to at least 10 times the highestdrive frequency connected to that chamber, that may be as much or morethan 300 MHz and may be capable of sensing the surface voltage, thesurface current, or both. The sensitivity of sensors 334 in someembodiments may vary by less than 30% over the range of frequencies ofthe harmonics of the principal fundamental RF frequency used in thereaction chamber. In some embodiments, at least one lead 337 from eachsensor is connected to the inner conductor 338 of a vacuum electricalsignal feedthrough 339, which has its base 341 mounted in theelectrically grounded second plate 215. In some embodiments the leadsfrom each sensor may be connected directly to a circuit board located insimilar position to 332 having a ground plane and detector circuits, onefor each sensor, to determine amplitude and phase for each frequencycomponent.

The inner conductor 338 of the feedthrough 339 may have a small shuntcapacitance to the base 341 of the feedthrough 339 mounted into thegrounded second plate 332—e.g., less than 5 pico-farads (pf, and in someembodiments less than 2 pf, such that the total shunt capacitance fromthe sensor 334 plus the lead 337 plus the feedthrough 339 to groundshould be less than 5 pf and in some embodiments less than 3 pf. In someembodiments the output from the base 341 mounted into the groundedsecond plate 332 may be connected to an attenuator (not shown). In someembodiments, the attenuator may include an electrical resistor having aresistance greater than about 100 Ohms. In parallel with the electricalresistor 404 there may be a shunt resistor to ground 405. The shuntresistor's resistance may be, e.g., 50 Ohms, or alternatively may beequal to the impedance of a cable that connects the attenuator to acommunications network or to a controller for the plasma chamber. Incases where the detectors are located in FIG. 3 instead of theconnectors as shown, the signal outputs from the detectors, which arethe amplitudes and phases of voltage or current at each frequency forthat sensor, may be transmitted to an analysis processor that may be ina compartment of the matching network.

Each sensor 334 may measure voltage or current amplitude of the combinedelectromagnetic surface wave modes, which have as components fundamentaland harmonic frequencies for all RF generators providing power to theplasma. The fundamental and range of harmonic frequencies ranging fromabout 10 kHz to as much as about 500 MHz or more. In other embodiment,the sensors may measure voltage at fundamental and harmonic frequenciesin range from about 100 kHz to about 1 Ghz.

FIG. 4 shows a cross-sectional view of a pedestal with an embeddedbroad-band voltage sensor, according to embodiments of the presentdisclosure. Voltage sensor 401 may be mounted into an electrode, such aspedestal 400. In some embodiments, the sensors 401 may be connectedthrough a resistor to electrical ground 406. The tip or puck of sensor401 may have a lead 402 surrounded by a dielectric 403 (which mayoptionally be air or vacuum). In some embodiments, the lead 402 from thesensor 401 may pass through an attenuator such as resistor(s) 404 with ashunt resistor 405 that may in some embodiments be about 50 Ohms and mayalso be connected to electrical ground 406. Such resistors 404 may benon-inductive and may have a resistance in the range between about 100Ohms and about 100,000 Ohms. In some embodiments the resistance may bebetween about 500 Ohms and about 10,000 Ohms. Resistor 405 may also benon-inductive.

Further, the dielectric 403 should be generally non-magnetic and have alow loss tangent, in some embodiments less than about 0.01 or in otherembodiments, less than about 0.001. The shunt capacitance between thetip of sensor 401 and lead 402 to the grounded electrode should be lessthan about 5 pf, or less than about 2 pf in some embodiments so that thereactance between the sensor 401 and the pedestal 400 electrode shouldbe greater than about 100 Ohms at 300 MHz. The purpose of such low shuntcapacitance is to reduce the loading of the surface wave by the sensor401 so that it minimally absorbs the wave energy and permits the wave topropagate as it would in the absence of the sensor 401. Under suchconditions, the surface potential that is detected will not be greatlydifferent than it would have been on an electrode without such sensors401.

Turning to FIG. 5, a schematic side view of a pedestal with associatedRF and control components, according to embodiments of the presentdisclosure is shown. Pedestal 501 power feed circuit includes RF powergenerator 405 and impedance matching network 506. High-speed signallines, e.g., cables 511, 512, carry signals from sensors 502, 503 to acompartment that in some embodiments may be in or attached to theimpedance matching network 506. High-speed lines 513 of a data networktake information from the impedance matching network 506 to thecontroller(s) 514 of the reaction chamber, or generator, or tool orfactory (not shown). Sensors 502, 503 are mounted on or near a base 504of pedestal 501, which may be inside or outside the vacuum region of areaction chamber.

In some embodiments, there may be a signal analysis, e.g.,fault-detection, compartment 510 associated with the impedance matchingnetwork 506. The signal analysis compartment 510 may be electricallyand/or RF isolated from certain components, such as vacuum capacitorsand high voltage electronics, of impedance matching network 506. Thesignal analysis compartment 510 receives signals from sensors 502, 503via cables 511, 12. Signal analysis compartment 510 then channels thesignals from each sensor 502, 503 to an internal circuit that may becalled a detector and may include electronic components such astransistors and passive components. In alternate embodiments where theamplitude and phase are found directly adjacent the sensor for eachfrequency component, the signals coming to the signal analysiscompartment may be amplitude and phase for each frequency componentrather than the raw signals.

Each detector (not shown) in the compartment 510 may perform RF spectrumanalysis of signals from one sensor 502, 503 or from a group of sensorsthat may be analyzed in parallel. The analysis may include averaging thesignals of a group of sensors, or of one or more sensors 502, 503 overtime, for noise reduction. There may in some embodiments be an outputfrom each detector of amplitude and phase for each frequency componentof the signal obtained by each sensor 502, 503, e.g., fundamental andharmonics. The outputs from each detector may then be input to ananalog-to-digital converter for each harmonic signal, yielding digitizedvalues for both amplitude and phase of each harmonic measured.

These digital amplitude and phase values for each frequency componentand each sensor may be input, with little to no delay, e.g., <10microseconds, to high-speed digital processors in the signal analysiscompartment associated with the disclosed impedance matching network.The digital processors may analyze both amplitude and phase informationfor fundamental and each harmonic from the sensors, determining therelative magnitude of the different surface-wave modes, including theaxisymmetric mode and non axi-symmetric modes, for both fundamental andharmonics. There may be differing non-axisymmetric modes for eachfrequency component, one or more of which may be indicators of theplasma non-uniformity.

In some embodiments, such non-axisymmetric modes may be rapidlyidentified by algorithms that reside on the plug-in. A referencedatabase correlating the magnitudes of non-axisymmetric modes withplasma non-uniformity percentages may also reside on this plug-in ordetachable processor. The digital processors may also compute rates ofchange of the amplitudes of the wave modes and acceleration ofamplitudes of one or more wave modes to determine the likelihood of afault in the immediate future. One measure of the magnitude of anon-axisymmetric mode at a given frequency may be the difference betweenthe phases of a given frequency surface wave at different sensorpositions which have the same radial distance from the center of acircular electrode, symmetrically located in an axisymmetric chamber.Alternatively, a second indicator of non-axisymmetric modes may bedifferences between the amplitudes of a given frequency surface wave atdifferent sensor positions which have the same radial distance from thecenter of a circular electrode that is symmetrically located in anaxisymmetric chamber.

A matching network 506 having an isolated compartment 510 containingmulti-channel detector systems (not shown) may simultaneously Fourieranalyze, digitize and record voltage amplitude and phase of EM wavespropagating at various locations on the pedestal 501. Because ofinherent noise, each of the determined voltage amplitudes and phases maybe averaged over brief time intervals, as needed, and may be averagedfor groups of sensors 502, 503 to make a determination of relativemagnitudes or average in time over a relatively large number of pulses.

A showerhead, pedestal, or other powered element such as an electrode,equipped with groups or arrays of sensors may be used as a test systemto generate data to characterize and record the relationship betweenspectra and spatial patterns of EM wave modes and variousnon-uniformities of the plasma density during an RF process. These datamay in some embodiments be analyzed offline by engineers to characterizeand categorize plasma behaviors and put into a database that may bestored in a plug-in storage device that may be connected to matchingnetwork compartment or other controller or monitoring systems.

The relationship between amplitude and phase pattern characteristics ofnon-axisymmetric and axisymmetric EM modes, and process and plasmanon-uniformities or deviations from proper conditions may be stored inthe plug-in that connects to the disclosed signal analysis compartmentof the matching network. In implementations where the RF plasmaprocessing system may be used as a production tool, the non-uniformityof the plasma and process may thereby be rapidly detected as theoperation of the chamber is being monitored. For example, the disclosedtype of sensor shown in FIG. 4, configured as shown in FIG. 2, may beretrofitted to a RF plasma system as shown in FIG. 1.

To determine whether the process plasma may have experienced a plasmafault condition, the analysis processors in the signal analysiscompartment associated with the impedance matching network may computeparameters based in part on the magnitudes of non-axisymmetric EM modesfor each of a pre-specified set of harmonics of a drive frequency onsome electrode or antenna. The processors in some embodiments may thencompare these parameters to reference ranges in a database. Suchreference database may reside on a plug-in that is connected to thesignal analysis compartment that may be a compartment in or associatedwith the impedance matching network.

The database may store parameters characterizing various plasmaconditions to aid in determining whether and how severe a plasmaexcursion from an acceptable “process window” is. In some embodiments,the analysis may include a comparison of phases of each harmonic fromevery sensor or group thereof at a given distance from the center of theelectrode. The variance of such phases for a sensor or group of sensorsabout any azimuth may be a measure of the asymmetry of the generationand/or propagation of that harmonic mode, and therefore may be a measureof plasma asymmetry and non-uniformity. In some embodiments the analysismay include calculation of differences of amplitude among sensors orgroups of sensors at a given distance from the symmetry axis. Thevariance of such amplitudes for a sensor or group of adjacent sensors ina range of azimuth may also be a measure of the asymmetry of thegeneration and/or propagation of that harmonic mode, and therefore maybe a measure of plasma asymmetry and non-uniformity.

A quantitative measure of the asymmetry for each of a set of harmonics,a parameter, may then be stored in the plug-in unit, and may betransmitted through the data network to the chamber and the toolcontrollers. Further, the trend and acceleration in the parameters maybe computed and compared with reference values and criteria in thedatabase as part of the process of determining whether a fault conditionoccurs. In some embodiments, when such fault condition occurs,algorithms and criteria that may be stored on the plug-in, may executein the processors resident in the compartment to determine a course ofremedial or preventive action. Such action then may be transmittedrapidly to the RF generator and/or the chamber and/or tool controllers.

In some embodiments, all such databases of parameters, algorithms,criteria, and specifications for comparing the parameters, rates ofchange of parameters, and acceleration of parameters may reside on adata storage device or detachable processor, that may be connected to aport that may be an input/output port of the signal analysiscompartment. The analysis of the surface wave modes based on signalsfrom the sensors, and parameters derived therefrom, performed so rapidlyby the processors that any fault declaration and remedial action ordersmay be transmitted to the RF generator, and reported to the controllerfor the chamber or system via a network within five milliseconds or lessof the occurrence. In some embodiments, a fault condition and specifiedremedial action orders may be transmitted to the generator within onemillisecond.

In some embodiments, many types of plasma excursions from desired plasmauniformity may be detected quickly enough that the tool or chambercontroller may take measures to correct the plasma fault conditionbefore the wafer or substrate is mis-processed. In some circumstances,the specified remedial action may be that RF power format, e.g.,continuous-wave (CW) or pulsed, is altered briefly, or power turned offcompletely for a brief period or frequency change, or processing of thecurrent wafer may be halted and the wafer saved for later processing ordiscarded, or the reaction chamber may be shut down for maintenance.

In certain embodiments, upon detection of a plasma fault condition, thedisclosed signal analysis compartment associated with the matchingnetwork may order appropriate corrective actions to be executed by theRF generator and/or in some embodiments by the matching network. Forexample, the RF process generator may initiate a termination process toend the processing of wafers in response to the signals measured bysensors on the showerhead and/or pedestal. Alternatively, power may beinterrupted, e.g., the institution of pulsed power, by the RF plasmaprocessing deposition system to stop or pulse the plasma so thatsecondary plasmas are stopped or greatly reduced. In some cases, after avery brief interruption, the specified remedial action may provide thatprocessing can then continue. In certain implementations, the remedialaction may be determined through, for example, machine learning and/orprogrammed remediation programs based on yield data or other waferdiagnostics.

Turning to FIG. 6, a top view of the propagation of axisymmetric surfacewaves across a pedestal where the plasma in the reaction chamber isaxisymmetric, according to embodiments of the present disclosure isshown. In FIG. 6, circles 601 are the curves of constant phase andamplitude for fundamental and harmonic frequency components ofaxisymmetric surface-wave modes. The circles are concentric with theelectrodes. These modes are highly dominant when in the chamber,electrode and plasma are all axisymmetric and coaxial. The propagationvectors 602 for the surface waves at any frequency will be radial. Thewaves will propagate both toward the center and away from the center,and as they propagate, such waves will inject power into the plasma.

Turning to FIG. 7, a top view of transverse electromagnetic surface wavepropagation across an electrode, according to embodiments of the presentdisclosure is shown. In FIG. 7, the lines 701-704 of constant phase andequal amplitude for a particular single non-axisymmetric mode areapproximately straight and parallel, whether at the fundamentalfrequency or a harmonic thereof. Such surface waves may be detected bysensors disposed on a pedestal or showerhead of the RF plasma depositionsystem. This mode may be called “transverse” which means that thedirection of propagation, as seen in propagation vectors 705-707, isacross the electrode surface from one side to the other or from acentral plane to both left and right sides. There may be othernon-axisymmetric modes where the lines of constant phase may be curvesthat have a center of curvature displaced from the center of theelectrode. The detector readings for each frequency can be decomposedinto a sum of axisymmetric modes and (often a small number of)non-axisymmetric modes that reflect the major non-uniformities of theplasma. Typically, the decomposition may permit identification of atransverse mode component and/orone main “off-center” or displacedradial mode, either of which is characteristic of a configuration ofplasma non-uniformity. The correlations of the configuration of plasmanon-uniformity with the particular non-axisymmetric modes is done inadvance of production processing as part of building the database, whichmay reside on the plug-in unit or elsewhere.

Turning to FIG. 8, a top view of one exemplary azimuthal sensordisposition for a reaction chamber, according to embodiments of thepresent disclosure is shown. In this embodiment, a plurality of sensors800 may be disposed azimuthally around one or more components of areaction chamber and/or on the reaction chamber itself. As brieflydiscussed above, the plurality of sensors 800, which in this embodimentmay be four, may be positioned upon certain chamber components, such asa showerhead and/or a pedestal, at different angles about a chambersymmetry axis 805 for measuring the surface voltage or currentassociated with surface waves. In this case at 90-degree intervals, butin some embodiments may be at irregular intervals of azimuth.

Sensors 800 may include passive sensors 800 that pick up changingelectric potentials or magnetic fields. Sensors 800 may be disposed atdiffering azimuths for detecting EM waves having different types ofpropagation modes relative to chamber symmetry axis 805. Sensors 800 maybe disposed at equidistant locations around chamber symmetry axis 805and/or components within a reaction chamber, or the reaction chamberitself. Similarly, sensors 800 may be disposed diametrically oppositeone another, such that the spacing between sensors 800 and the symmetryaxis may be approximately the same. For example, the distance betweensensor 800-1 and 800-2 is approximately the same as that between 800-3and 800-4. Similarly, each sensors 800 is located the same distance fromchamber symmetry axis 805. Examples of sensor 800 spacing and locationare discussed in greater detail below.

As illustrated, the sensors 800 are disposed at diametrically oppositelocations. For example, sensor 800-1 is diametrically opposed to sensor800-3, while sensor 800-2 is diametrically opposed to sensor 800-4. Thesensors 800 may for non-axisymmetric plasma thus find differences inwave forms on different sides of the reaction chamber and/or componentsthereof, and when differences in the wave forms occur, providenotification, as explained above, so that remedial or proactive actionsmay be taken. For example, if sensor 800-1 and sensor 800-4 sense andreport a difference in waveform from their diametrically opposedlocations, such differences may provide an indication that the harmonicsare out of phase or have differing amplitudes, which may therebyindicate there is plasma nonuniformity and asymmetry. Such differencesin waveform occur when there are differences between diametricallyopposite detectors in the relative phase or amplitude of one or moreharmonics in signals picked up by the opposite sensors.

In certain embodiments, four sensors 800 may be used, as illustrated inFIG. 8. However, in other embodiments differing numbers of sensors 800,such as six, eight, twelve, fourteen, sixteen, eighteen, twenty, or moresensors 800 may be used. In some embodiments the azimuth angles betweensensors may not be equal, nonetheless, the same characteristics ofnon-azimuthally symmetric plasma modes may be observed by sensors. Incertain implementations, it may be beneficial to have between six andtwelve sensors 800. The greater the number of sensors 800, the more datamay be collected, thereby providing enhanced ability to discriminateagainst noise and sensitivity for recognition of nonuniformity. However,by increasing the number of sensors 800, data processing may be slowed,thereby resulting in remediation and preventive actions that occur moreslowly. Those of ordinary skill in the art will appreciate thatbalancing the number of sensors 800 with a desired level of granularityof data may thereby allow the RF plasma process to be optimized. Assuch, as computing power increases, and the speed with which data may beprocessed increases, it may be beneficial to increase the number ofsensors 800. In certain embodiments, specific sensors 800 may beselectively turned off and on, thereby allowing controllers to accesscertain desired data. For example, in a system having eight sensors,four of the sensors may be selected and turned off, thereby decreasingthe amount of generated data. In other embodiments, additional sensorsmay be added or removed from operation, thereby changing the amount ofdata that is generated.

Sensors 800 may also include various types of sensors, both round andother geometries. In certain embodiments, sensors 800 may be circularhaving an area between about 0.1 square centimeter and about 10 squarecentimeters. Sensors 800 may further include a surface insulator layeror coating to protect sensors 800 from plasma or reactive species in areaction chamber and may also include other optional coatings and layerssuch as faraday shields for current sensors, aluminum coatings, and thelike.

Turning to FIG. 9, a side cross-sectional view of azimuthally mountedsensors on a reaction chamber, according to embodiments of the presentdisclosure is shown. In this embodiment, reaction chamber 940 has asymmetry axis 905 that runs longitudinally from the center of showerhead930 through pedestal 935. In other embodiments, symmetry axis 05 may runlongitudinally from the center of another electrode, such as an antenna.A plurality of sensors 900 may be azimuthally disposed at variouslocations around and inside reaction chamber 940, as well as around orassociated with specific components, such as showerhead 930 and/orpedestal 935. As FIG. 9 is a cross-section, only two sensors 900 foreach location are illustrated, however, more sensors 900 may be usedduring implementation of the RF plasma monitoring process, as discussedin detail with respect to FIG. 8.

In certain embodiments, sensors 900-1 may be disposed around the edge orperiphery of showerhead 930. In such an implementation, sensors 900-1may be disposed at least partially or completely embedded withinshowerhead 900-1 and the outer surface of sensors 900-1 may be coatedwith an insulating layer, thereby protecting sensors 900-1 from theenvironment within reaction chamber 940. In such an embodiment, two ormore sensors 900-1 may be azimuthally disposed around the edge ofshowerhead 930, and preferably four or more sensors thereby allowing fordetection of nonuniformity and asymmetry in RF plasma processing.

In other embodiments, sensors 900-2 may be disposed along the edge ofpedestal 935 within the vacuum of reaction chamber 940. As explainedabove with respect to sensors 900-1, sensors 900-2 may be partially orcompletely embedded in pedestal 935 and may or may not include aninsulating layer disposed on an outer surface thereof. Further, in someembodiments they may have a dielectric protective part covering them. Inadditional to sensors 900-2 disposed around the pedestal 935 inside thevacuum, other sensors 900-3 and 900-4 may be disposed outside of thevacuum of reaction chamber 940 and around pedestal 935. Such sensors900-3 and 900-4 may be disposed on a metal surface along pedestal 935and/or a base portion thereof. Sensors 900 may also be disposed on othersupport structures of or associated with pedestal 935.

In still other embodiments, sensors 900-5 may be disposed and/orotherwise built into the sidewall of reaction chamber 940. In suchembodiments, where the wall is dielectric, sensors 900-5 may be disposedoutside reaction chamber 940 on an outside chamber wall 915 or may bebuilt into the sidewall so that sensors 900-5 are within the vacuum ofreaction chamber 940. For metal walls, sensors should have their pickupsexposed in the inner surface of the wall so that EM fields on the insideof the chamber may be sensed. Other sensors 900-6 may be disposed intoview ports 920, which are located along the outside chamber wall 915. Insuch embodiments, sensors 900-6 in viewports may be located outside thevacuum of reaction chamber 940 or located within reaction chamber 940.

In yet other embodiments, sensors 900-7 may be disposed in dielectriclocated around, for example showerhead 930, while in otherimplementations, sensors 900-7 may be disposed in dielectric locatedaround pedestal 935. While specific locations for sensors 900 arediscussed herein, sensors 900 may be located at various other locationsin and around reaction chamber 940. For example, sensors 900 may bedisposed inside or outside a dielectric wall near an antenna or othercomponents. Sensors 900 may further be located at various otherlocations inside a metal wall of reaction chamber 940.

In certain embodiments, combinations of sensors 900-1-900-7 may be usedin order to more accurately monitor RF plasma processing. For example,sensors 900-1 around the edge of showerhead 930 may be combined withsensors 900-2 around the edge of pedestal 935. Similarly, combinationsof sensors 900-5 outside reaction chamber 940 may be combined withsensors 900-1/900-2 located within reaction chamber 940. In still otherembodiments, combinations three, four, five, six, seven, or morevariations of sensor 900 location may be used to further optimizemonitoring of RF plasma processing.

Turning to FIG. 10, a side cross-section of a model reactor chamberaccording to embodiments of the present disclosure is shown. In thisembodiment exemplary positions are shown for a plurality of azimuthallydisposed sensors 1000, located around a bottom electrode which in thisexample is pedestal 1035. Similar to the sensors 1000 discussed abovewith respect to FIG. 9, FIG. 10 illustrates sensors 1000 that aredisposed in various locations. Sensors 1000-1 are disposed around anouter edge of pedestal 1035. Sensor azimuthal positions indicated as1000-2 are disposed around the inside of reaction chamber 1040, whilesensor azimuth positions 1000-3 are disposed around the outer peripheryof reaction chamber 1040 adjacent viewports.

In this embodiment, twelve sensors 1000 are illustrated at eachlocation, however, in other implementations, other numbers of sensors1000, both fewer and greater, may be used. Also, in addition to thesensor 1000 locations expressly illustrated, other sensor 1000 locationsmay also be used to further enhance RF plasma processing.

Turning to FIG. 11, a side schematic cross-section of a reaction chamberaccording to embodiments of the present disclosure is shown. In thisembodiment, sensors 1100 are illustrated disposed around the antenna ofan inductively coupled plasma source 1105. As such, sensors 1100 maysense RF currents or voltages from a plasma source that is locatedwithin reaction chamber 1140.

Turning to FIG. 12, a partial cross-section of a RF plasma processingsystem, according to embodiments of the present disclosure is shown. Inthis embodiment, RF plasma processing system 1200 includes a pedestal1235. Pedestal 1235 includes sensors 1240 that are disposed along anupper outer edge of pedestal 1235. As described above, sensors 1240 maybe disposed on an upper, outer edge, embedded within pedestal 1235, ormay alternately be disposed around an outer edge either inside oroutside the vacuum of a reaction chamber.

RF plasma processing system 1200 also includes circuitry 1245 that isconnected to sensors 1240 through communication lines 1250. As sensors1240 receive sensed data from RF plasma processing system 1200, the datamay be sent to circuitry 1245 for processing. Because the circuitry 1245is relatively close to sensors 1240, the time taken to transfer thesensed data therebetween may be decreased. As such, initial calculationsas to the electrical properties sensed by sensors 1240 may be performedmore quickly, then transferred to other components 1255 of RF plasmaprocessing system 1200. The other components 1255 may include, forexample, an RF generator, an impedance matching network, a faultdetection compartment, an operation controller for the reaction chamber,an operational controller for the tool, a plug-in device, a signalanalysis compartment, or other component(s) connected to RF plasmaprocessing system 1200.

Components of the RF plasma processing system 1200, either the othercomponents1255 or still other components not shown may then adjustaspects of RF plasma processing system 1200 to correct for a fault thatis detected by sensors 1240 and processed at least partially withincircuitry 1245. Circuitry 1245 may be located within pedestal 1235outside of the vacuum of the reaction chamber (not shown) in an isolatedstructure to protect circuitry 1245 from the conditions within thereaction chamber. In other embodiments, circuitry 1245 may be located ina base of pedestal 1235, or in other areas proximate pedestal 1235.

As FIG. 12 illustrates a cross-section of a components of an RF plasmaprocessing system 1200, one of ordinary skill in the art having thebenefit of this disclosure will appreciate that circuitry 1245 may bedisposed at approximately the same radius at different azimuths aroundpedestal 1235. As such, independent circuitry 1245 may be available foreach sensor 1240 or sensors 1240 and may be connected to centralizedcircuitry 1245 that is located in one or more select locations aroundand/or within pedestal 1235.

Turning to FIG. 13, a partial cross-section of an inductively coupled RFplasma processing system 1300, according to embodiments of the presentdisclosure is shown. Sensors 1340 are shown configured proximate theinduction antenna 1330 and may be mounted outside or inside a dielectricwall (not shown) that is adjacent the antenna.

Turning to FIGS. 14, 15, and 16, show several process phases in an RFplasma processing system matching network, according to embodiments ofthe present disclosure are shown. In FIG. 14, phase one of a process isshown, wherein the amplitude of the radio frequency increases with timeas the DC power, feeding the RF section, is brought to full power whenit is turned into an on position, thereby providing a higher voltage.Phase one may last, for example, approximately 1 millisecond, dependingon the design features provided by the manufacturer. As illustrated, theforward voltage 1400 is increasing with each pulse, and similarly, thereflected voltage 1405 is also increasing with each pulse.

In FIG. 15, phase two of a process is shown, wherein the amplitude ofthe radio frequency voltage is constant, but the match is not turned.Accordingly, the amplitude of the forward voltage 1500 is constant, asis the amplitude of the reflected voltage 1505. In FIG. 16, phase threeof a process is shown, in which the match is tuned. In phase three, theamplitude of the forward voltage 1600 is constant. The amplitude of thereflected voltage 1605 is also constant, however, the amplitude of thereflected voltage 1605 is also lower. A fourth phase, not illustrated,may also occur in some embodiments, in which a process parameter haschanged, such as a change to a capacitor, and an end point is reached.During the fourth phase, an impedance may have changed, which may resultin a change to the voltage.

Turning to FIG. 17, a schematic representation of the phases of a radiofrequency processing system matching network, according to embodimentsof the present disclosure is shown. As discussed in greater detailabove, a process may include four phases. During phase one 1700, thedirect current is ramping up. During phase two 1705, the matchingnetwork begins to become tuned, but may not be fully tuned. In phasethree 1710 a steady state may occur, in which the reflected voltage isrelatively low. In the fourth phase 1715, an end point may be reached.This process may occur numerous times for a particular wafer.

Aspects of the present disclosure may allow for statistics to be derivedfor individual wafers based on dynamic changes in the process that arerepresented above with respect to the various phases of operation.Accordingly, statistics may be performed for one or more of the phases,and may include signals associated with a particular pulse. For example,statistics may be prepared that allows for voltage of a radio frequency,current of a radio frequency, and/or phases in terms of degrees betweenthe voltage of the radio frequency and the current of the radiofrequency. The process may be completed for numerous wafers, and thestatistics compiled, thereby allowing for trends to be monitored. Thetrends may be used to determine when various alerts and/or interventioncommands may be issued. Aspects of determining the statistics andgenerating alerts and intervention commands are discussed in detailbelow.

Turning to FIG. 18, a graphical illustration of voltage at a beginningof a radio frequency pulse in a radio frequency plasma processingsystem, according to embodiments of the present disclosure is shown.Before discussing the accumulation of statistical data and how the datais used, an example of how a plus reaches a steady state is illustrated.In this illustration, the x-axis represents time in microseconds and they-axis represents a unit measurement proportional to voltage. Thus,various measurements and calculations, such as slope, time to reach maxvoltage, time to reach steady state max voltage, and voltage at steadystate. Such measurements may be performed for each and/or a plurality ofpulses, which may thereby allow for averages for particular values,standard deviations for particular values, ramps, trends, and the liketo be monitored. The acquisition and use of such measured values willnot be discussed in detail. Using the above systems and methods,additional aspects of the present disclosure may allow for controllingthe time it takes to establish plasma and the duration of the afterglowof the plasma. Control of the transients of turning on and off the RFpower allows for more repeatable and reliable processing of wafers in aRF plasma processing system. For a well-established process, while theprocess may be relatively similar, the user obtains increasedrepeatability. Modifying a process may thereby make the transientduration change. Accordingly, separating both effect of time and effectof voltage allows for a clear picture of what is required to result in asuccessful process.

During normal operation power is provided from a generator to a matchingnetwork, as discussed in detail above. When power is turned off, such asduring pulse processing, there is a greater amount of reflective powerthan forward power, so power is drawn back into the generator. Thiscondition results in the chemistry, e.g., a mix of species that ispresent such as, for example, ions, radicals, atoms, and/or moleculesabout a wafer being different than the chemistry of the steady state ofthe pulse, which may be managed to result in beneficial consequences forthe plasma process. The chemistry is different during the steady stateof the pulse than during the afterglow, which makes pulsing beneficial,as the mix of particles is different during the different phases.Additionally, the period of time between when the power is turned off atthe generator and when the residual power has left the matching networkis not controlled. By controlling this period of time, a more stableprocessing environment may be established.

During operation, certain systems and methods may allow for changing theresonating aspects of the match at the time the RF is turned off. Toadjust the resonating aspects to acceptable values, at approximately thetime of an end of an RF pulse, the output of the match may be switchedrelatively quickly, such as within ten microseconds or less, to a knownpredicted additional load, either fixed or variable, such that decay ofaccumulated RF energy in the match will take a period of time that isdefined by a user. In certain aspects, the output of the match may beswitched even more quickly, such as less than seven microseconds, lessthan five microseconds, less than three microseconds, or less than aboutone microsecond. In certain implementations, the time the output isswitched to a known additional load may occur at the end of the RFpulse, slightly before the end of the RF pulse, or slightly after theend of the RF pulse, depending on the requirements of the RF processingsystem.

Providing an additional load may thereby provide for a RF voltage on aplasma or wafer that matches a predetermined level of voltage that isdesirable within a specific RF processing system. The desirable voltagemay vary based on the operational dynamics of the system, preferences ofa user, manufacturer of the RF processing system, components of the RFprocessing system, and other system and design variants.

In order to control the RF processing system to establish a desired RFvoltage for a desired time, initially a determination of an end time ofa radio frequency pulse may occur as seen in the reaction chamber. Theradio frequency pulse includes a load that is applied within the radiofrequency processing system, such as within a reaction chamber inaccordance with the embodiments discussed above. The end time definesthe time when the RF power is stopped, thereby changing the voltagebeing applied within the reaction chamber. In certain implementations,the end time may correspond to the end of a pulse and may include aslight delay from when power is turned off, such as a number ofmicroseconds. As such, end time may be temporally relative depending onthe operational dynamics of a specific RF processing system and/orcomponents thereof.

In operation, the method may further include stopping the power appliedto the RF processing system. “Stopping the power” refers to removing theapplication of voltage within the reaction chamber by, for example,turning off a generator connected to the reaction chamber.

In operation, the method may further include adjusting an additionalload having a predetermined impedance applied to the radio frequencyprocessing system in response to the determined end time. The additionalload may be applied from a device separate from the generator or mayinclude an additional aspect of the generator. For example, theadditional load may be applied by adjusting an aspect of a switchable orvariable component, such as a capacitor, which may be run in series orparallel with one or more of the generator and the matching network.

In certain embodiments, adjusting the additional load may occur slightlybefore the end point, such as several microseconds, 10 microseconds,10-50 microseconds, or greater than 50 microseconds. Similarly, in otherembodiments the adjusting the additional load may occur slightly afterthe end point, such as several microseconds, 10 microseconds, 10-50microseconds, or greater than 50 microseconds. The timing of theadjusting the additional load may be based on operational aspects of theRF processing system, user input, manufacturer input, requirements of aprocess, and the like.

In operation, the method may further include determining a start pointof a second RF pulse. The start point defines the beginning of theapplication of a load to the reaction chamber. The start point for thesecond RF pulse may occur microseconds after the end time of the firstRF pulse. For example, the start point for the second RF pulse may occurseveral microseconds, 10 microsecond, 10-50 microseconds, 50-100microseconds, or more than 100 microseconds after the end time of thefirst RF pulse. In certain implementations, the start point for thesecond RF pulse may occur 5 milliseconds, 10 milliseconds, 50milliseconds, 100 milliseconds, more than 100 milliseconds from the endtime of the first RF pulse.

Moreover, the determination of the start point may be dependent on anumber of variables including, for example, the type of process, thetype of generator, the type of matching network, the type of RFprocessing system, the requirements of a user, the requirements of theoperation, the requirements of a manufacturer, and the like.

During operation, the method may further include stopping the additionalload before the second radio frequency pulse occurs. When the conditionwithin the reaction chamber reaches a desired state, the additional loadmay be removed, thereby allowing the condition within the reactionchamber to stabilize to the desired state. At a predefined period oftime after the additional load is stopped, a second radio pulse havinganother load may be started within the reaction chamber, and the processdefined above may be repeated until the processing operation iscomplete.

In certain implementations the value of the additional load may also bedetermined. In such an implementation, a method for determining thevalue of the additional load may include determining a plasma impedancewhen the load is disconnected. The value of the additional load may thenbe set to equal the determining plasma impedance.

With the value of the additional load set to equal determining plasmaimpedance, and before the power is turned on, the output of the matchingnetwork may be disconnected from the plasma and the additional load maybe connected to the output of the matching network. The RF voltage maythen be turned on. After a predefined time that the RF voltage is on,the plasma powering electrode may be switching on to the output of thematching network. At approximately the same time, the additional load isswitched off. This process may be repeated to observe the transitionfrom load to plasma.

At this point, the values of resistance and reactance of the impedancemay be varied until the RF plasma processing system is optimizedaccording to the requirements of, for example, a user, a process, andmanufacturer, or the like. In certain implementations, the values ofresistance and reactance of the impedance may be varied by approximately10 percent, 10 to 20 percent, 20 to 50 percent, or greater than 50percent.

Once the values of resistance and reactance of the impedance areestablished, production of wafers may continue, and subsequent switchingmay be performed by internal controls of the matching network.

Referring to FIG. 19, a cross-sectional view of a RF processing systemaccording to embodiments of the present disclosure is shown. In thisembodiment, which may be used according to the methods described above,the RF processing system 1900 includes a reaction chamber 1905 and amatching network 1910 for applying a RF voltage to reaction chamber1905. In this embodiment, a number of parallel components 1915 andseries components 1920 are illustrated that may be used to provide anadditional load to reaction chamber 1905. Examples of components thatmay be used to apply the additional load include, for example, variablevacuum capacitors, electronically variable capacitors, and pin-diodeswitchable capacitors. The additional load (not represented) may belocated in the matching network 1910 between a ground and a switch 1930,or a diode 1935, or a fast-acting capacitor. This load can be locatedexternally to matching network 1910, internally within matching network1910on reaction chamber 1905, about a powering electrode (not shown),connected by a switch 1930, a diode 1935, or a fast variable capacitor.Those of skill in the art having the benefit of this disclosure willappreciate that any type of component that may apply the additional loadto reaction chamber 1905 may be used according to embodiments disclosedherein.

Referring to FIG. 20, a RF and DC decay process diagram for a RF plasmaprocessing system matching network, according to embodiments of thepresent disclosure are shown. The process diagram illustrates the RFdecay 2000 during the end of a RF pulse. The process diagram furtherillustrates the DC decay 2005 at the end of a RF pulse. As discussedabove, the decay can be controlled by introducing an additional loadthat results in a decay pattern that is desirable for a particularprocess. For example, in certain implementation, it may be desirable toobtain a flat decay profile, while in other implementations the decaymay be controlled according to a predefined value, as discussed indetail above.

By adding switchable loads on the output of a matching network, the timethe RF power takes to leave the match may be modified, thereby allowingthe time for the plasma to extinguish to also me modified to a desiredtime.

Referring to FIG. 21 a schematic representation of where a switchablecomponent to add an additional load according to embodiments of thepresent disclosure is shown. As illustrated, a matching network 2200 isconnected to the rest of a RF plasma processing system 2205, whichcontains a reaction chamber 2210. In this embodiment, matching network2200 includes a switchable component 2215, that may be turned on an offto adjust an additional load applied within reaction chamber 2210. Inthis implementation, switchable component 2215 is connected after anumber of capacitors 2220, in line before exiting matching network 2200to RF plasma processing system 2205. In other implementations,switchable component 2215 may be located external to matching network2200 or at a different location within matching network 2200 circuitry.

In still other embodiments of the present disclosure, other methods ofcontrolling a RF processing system may be used. By changing the value ofone of the capacitors in a matching network away from an optimum tunedposition, the time the plasma reaches a steady state may be increased.Such an adjust may cause a higher reflected power, which can becompensated for by deliver higher power. The voltage on a waferelectrode may also be modulated so that the voltage on the wafer levelwill be substantially continuous for a period or otherwise have avoltage shape that is desirable to a user.

In operation, such methods may include determining a first capacitorvalue and a second capacitor value for a RF plasma process. Thecapacitor values may refer to the values provided to the RF plasmaprocessing system and may automatically be determined based onparticular aspects of the process, such as desired loads within thereaction chamber. The method further includes measuring a poweringelectrode maximum voltage and a wafer electrode maximum voltage.

After the values are determined and/or measured, the method may includechanging at least one of the first capacitor value and the secondcapacitor value to reach the wafer electrode maximum voltage.Subsequently, the method may include determining a control voltage wherethe control voltage is based on the changing at least one of the firstcapacitor value and the second capacitor value to reach the waferelectrode maximum voltage.

After the control voltage is converted, the control voltage may be sentto a generator to deliver more or less power when a determined voltageon the wafer is lower than a predefined value. The predefined value maybe defined in accordance with a user specification, a processrequirement, manufacturers suggestion, and the like. In certainembodiments the control voltage to the generator to deliver more or lesspower when a determined voltage on the wafer is lower than thepredefined value results in an approximately constant voltage applied tothe wafer. In other embodiments, sending the control voltage to thegenerator to deliver more or less power when a determined voltage on thewafer is lower than the predefined value results in a voltage on thewafer than varies as a function of time.

In operation more or less power may be delivered to the wafer when thevoltage is low, thereby resulting in a flat, i.e., constant, value ofvoltage on the wafer. In certain implementations, a user may determinethat rather than a flat voltage, it is more desirable to have a rampedvoltage or another type of process curve that is a function of time.

In certain typical implementations the matching network may include atank circuit. A tank circuit refers to a parallel combination of acapacitor and an inductor. Thus, when the RF voltage is turned on, itmay take a certain period of time for the matching network, in thisembodiment the tank circuit, to fill in. Another type of delay may occurwith the establish of the plasma. In typical RF plasma processingsystem, the cause of the delay may be considered not controllable.

Embodiments of the present disclosure may alleviate the delay issues insuch circuits by changing how fast the tank circuit is filled in. Saidanother way, the tank circuit can be filled in by connecting the outputof the matching network to a known fixed or variable load. Once the tankis filled in, the output can be switched from the load to the plasma.The double-switching, i.e., switching off the load and switching on theplasma may be either synchronous or asynchronous. The additional loadhas an impedance, i.e., resistance and reactance of the impedance, thatmay be selected to be approximately equivalent to the impedance of theplasma.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the disclosure.However, it will be apparent to one skilled in the art that the specificdetails are not required in order to practice the systems and methodsdescribed herein. The foregoing descriptions of specific examples arepresented for purposes of illustration and description. They are notintended to be exhaustive of or to limit this disclosure to the preciseforms described. Obviously, many modifications and variations arepossible in view of the above teachings. The examples are shown anddescribed in order to best explain the principles of this disclosure andpractical applications, to thereby enable others skilled in the art tobest utilize this disclosure and various examples with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of this disclosure be defined by the claims andtheir equivalents below.

What is claimed is:
 1. A method of controlling a radio frequencyprocessing system, the method comprising: determining an end time of aradio frequency pulse comprising a first load; stopping a power appliedto the radio frequency processing system based on the determined endtime of the radio frequency pulse; adjusting an additional load having apredetermined impedance applied to the radio frequency processing systemin response to the determined end time; determining a start point of asecond radio frequency pulse; and disconnecting the additional loadbefore the determined start point of the second radio frequency pulseand the second radio frequency pulse occurs.
 2. The method of claim 1,wherein the adjusting the additional load having a predeterminedimpedance applied to the radio frequency processing system in responseto the determined end time occurs before the end of the radio frequencypulse.
 3. The method of claim 2, wherein the adjusting occurs withinabout 50 microseconds before the end of the radio frequency pulse. 4.The method of claim 1, wherein the adjusting the additional load havingan impedance to have a response matching the determined end time.
 5. Themethod of claim 4, wherein the adjusting occurs within about 50microseconds after the end of the radio frequency pulse.
 6. The methodof claim 4, wherein the adjusting occurs within less than about 5microseconds after the end of the radio frequency pulse.
 7. The methodof claim 1, wherein the adjusting the additional load having apredetermined impedance to has a response matching the determined endtime changes a resonance in the radio frequency processing system. 8.The method of claim 1, further comprising determining a plasma impedancewhen the additional load is disconnected.
 9. The method of claim 8,further comprising setting a value of the additional load to be equal tothe plasma impedance.
 10. The method of claim 9, further comprisingvarying a resistance and a reactance of an impedance.
 11. The method ofclaim 10, wherein the varying comprises varying the resistance and thereactance of an impedance by about 10 percent or greater.
 12. A methodof controlling a radio frequency processing system, the methodcomprising: determining a first capacitor value and a second capacitorvalue for a radio frequency plasma process; measuring a poweringelectrode maximum voltage and a wafer electrode maximum voltage;changing at least one of the first capacitor value and the secondcapacitor value such that the voltage measure on the electrode reaches amaximum. determining a control parameter based on the changed of atleast one of the first capacitor value and the second capacitor value toreach the wafer electrode maximum voltage; and sending the controlvoltage to a generator to deliver more or less power when a determinedvoltage on the wafer is lower than a predefined value.
 13. The method ofclaim 12, wherein the sending the control voltage to the generator todeliver more or less power or an adjusted frequency to match apredetermined voltage on the plasma electrode or on the wafer is lowerthan the predefined value results in an approximately constant voltageapplied to the wafer.
 14. The method of claim 12, wherein the sendingthe control voltage to the generator to deliver an adjusted power or anadjusted frequency to match a predetermined voltage on the plasmaelectrode or on the wafer is lower than the predefined value results ina voltage on the wafer than varies as a function of time.
 15. A radiofrequency plasma processing system comprising: a reaction chamber forreceiving a radio frequency pulse, the reaction chamber comprising aload; a matching network electrically connected to the reaction chamber;a switchable component electrically connected to the matching network toadjust an additional load having a predetermined impedance applied tothe radio frequency processing system.
 16. The system of claim 15,wherein the switchable component comprises at least one of a variablevacuum capacitor, an electronically variable capacitor, and a pin-diodeswitchable capacitor.
 17. The system of claim 15, wherein the switchablecomponent is disposed externally to the matching network.
 18. The systemof claim 15, wherein the switchable component is disposed internallywithin the matching network.
 19. The system of claim 15, wherein theswitchable component adjusts the additional load within about 50microseconds after the end of the radio frequency pulse.
 20. The systemof claim 15, wherein the switchable component adjusts the additionalload with less than about 5 microseconds after the end of the radiofrequency pulse.